Tocotrienols are vitamin E-related compounds whose occurrence in plants is limited primarily to the seeds and fruits of most monocot species (e.g., palm, wheat, rice and barley). Tocotrienols are structurally similar to tocopherols, including α-tocopherol or vitamin E, which occur ubiquitously in the plant kingdom as well as in photosynthetic microbes such as Synechocystis. 
Tocotrienols and tocopherols both contain a chromanol head group that is linked to a hydrocarbon side chain. The only structural difference between these molecules is the presence of three double bonds in the hydrocarbon side chain of tocotrienols. This difference is related to the biosynthetic origins of the side chains. Tocopherol side chains are derived from phytyl-pyrophosphate (PP), and the tocotrienol side chains are believed to be derived from geranylgeranyl-PP, see FIG. 1 and FIG. 2, respectively (Soil et al. (1980) Arch. Biochem. Biophys. 204:544-550).
At least four forms or molecular species of tocopherols and tocotrienols occur in nature: alpha, beta, gamma and delta (α, β, γ and δ, respectively). These molecular species contain different numbers of methyl groups that are bound to the aromatic portion of the chromanol head. Like tocopherols, tocotrienols are potent lipid-soluble antioxidants and therefore have considerable nutritive value in human and animal diets (Packer et al. (2001) J. Nutr. 131:369 S-373S). In addition, tocotrienols are believed to have therapeutic properties including a demonstrated ability to down regulate cholesterol biosynthesis (Theriault et al. (1999) Clin. Biochem. 32:309-319; Qureshii et al. (1986) J. Biol. Chem. 261:10544-10550).
The first committed step in the tocopherol biosynthetic pathway is the prenylation of homogentisic acid with phytyldiphosphate to form 2-methyl-6-phytylbenzoquinol (MPBQ). Two distinct methyltransferase enzymes catalyze methylations of the aromatic moiety of tocopherols (VTE3 and VTE4). 2-methyl-6-phytylbenzoquinol methyltransferase (VTE3) acts on the tocopherol intermediate MPBQ prior to cyclization. Cyclization of the product of the first methylation reaction (2,3-dimethyl-5-phytylbenzoquinol) with tocopherol cyclase (VTE1) provides gamma-tocopherol. Gamma-tocopherol is further methylated to alpha-tocopherol by the second methyltransferase enzyme of tocopherol biosynthesis, gamma-tocopherol methyltransferase (VTE4). The same enzyme methylates delta-tocopherol thereby generating beta-tocopherol.
It has been speculated that the first committed step in the biosynthesis of tocotrienols involves the condensation of geranylgeranyl-PP and homogentisate to form 2-methyl-6-geranylgeranylbenzoquinol (Soil et al. (1980) Arch. Biochem. Biophys. 204:544-550). The enzyme that catalyzes this reaction can thus be functionally described as a homogentisate geranylgeranyl transferase (HGGT). After cyclization and an initial methylation, the last step of tocotrienol production would require the methylation of gamma-tocotrienol to alpha-tocotrienol or delta-tocotrienol to beta-tocotrienol.
Functional identification of genes or cDNAs encoding homogentisate geranylgeranyl transferase (HGGT) and gamma-tocopherol methyltransferase polypeptides has been reported. However, the use of these nucleic acids in combination to manipulate the biosynthesis of the nutritionally important tocotrienols, such as alpha- and beta-tocotrienol, in plants, seeds and microbial hosts has not yet been reported.